Conventionally, methods such as vacuum evaporation, sputtering, CVD, plating and metal paste method have been known as a method for forming a metal thin film onto a substrate. Among these methods, vacuum evaporation, sputtering and CVD all suffer from the problem that they require an expensive vacuum chamber, and that their film deposition rates are slow.
Plating methods can relatively easily form a metal thin film on substrates that are conductive. However, when forming a thin film on an insulating substrate, plating requires first forming a conductive layer, which makes the process cumbersome. Plating methods suffer from another problem in that because they rely on a reaction in solution, a large amount of waste liquid is generated, which takes a lot of effort and cost to clean up.
A metal paste method is a method which involves applying a solution in which metal particles are dispersed onto a substrate, followed by heat treatment to give a metal thin film. This method has the advantages that it does not require special equipment such as that of vacuum evaporation and the like, and the process is simple. However, to fuse the metal particles, the process usually requires a high temperature of 1000° C. or more. Thus, the substrates are restricted to those having heat resistance such as ceramic substrates, and there still remains the problem that the substrate will be susceptible to damage from heat or residual stress caused by the heating.
On the other hand, there is a known technique for lowering the baking temperature of a metal paste by decreasing the particle diameter of the metal particles. For example, JP-B-2561537 discloses a method for forming a metal thin film using a dispersion into which metal fine particles having a particle diameter of 100 nm or less are dispersed. However, because the method for producing the necessary metal particles having a particle diameter of 100 nm or less is a method which involves rapid cooling of metal vapor vaporized under low pressure, this method suffers from the problems that mass production is difficult and that the cost of the metal particles is very high. A method which involves forming a metal thin film using a metal oxide paste which has metal oxide particles dispersed therein is also known. A method is disclosed in JP-A-05-98195 which involves heating a crystalline polymer-containing metal oxide paste into which a metal oxide having a particle diameter of 300 nm or less is dispersed, whereby the crystalline polymer is decomposed to obtain a metal thin film. However, this method requires previously dispersing the 300 nm or less metal oxide into the crystalline polymer, which in addition to requiring a lot of effort, also requires a temperature of 400 to 900° C. to decompose the crystalline polymer. Thus, this method has the problem that usable substrates require heat resistance above that temperature, which places a restriction on the substrates that can be used.
From the above, it can be seen that a method for obtaining a metal thin film by applying onto a substrate a dispersion into which metal particles or metal oxide particles are dispersed, then subjecting to heat treatment, has low process costs. However, methods that use metal particles have the drawback that the particle costs are very expensive. In addition, methods that use metal oxide particles require that the particles have been dispersed in the crystalline polymer beforehand, which has the problem that the heating temperature is high in order to burn off the crystalline polymer. Therefore, at present such methods are not used in practice. In particular, at present it is difficult to apply the methods for forming metal thin film onto the resin substrates used in the commercial sector. Meanwhile, it is difficult to obtain a porous metal thin film by heat treatment at a relatively low temperature.
Plating and the slurry method are also known as methods for producing metal oxide porous films. Plating is a method for obtaining a metallic porous body by adhering a carbon powder or the like to the skeleton structure surface of a foam resin such as urethane foam to provide conductivity thereto, depositing a metal layer thereon by plating, then burning off the foam resin and carbon powder to obtain the metallic porous body. However, this method has the drawback that its production step is cumbersome. On the other hand, the slurry method is to obtain a metallic porous body by impregnating or applying a metal powder or metal fiber to the skeleton structure surface of a foam resin such as urethane foam, and thereafter burning off the resin component by heating to sinter the metal powder and obtain the metallic porous body. This method requires that the metal powder or metal fiber, which are its raw materials, are small particles in order to impregnate the porous resin. Generally, particles having a diameter of several tens of μm to several hundreds of μm are used. However, producing a metal powder having a small particle diameter requires a cumbersome production step such as that used in a method of spraying a molten metal or a pulverizing method. Therefore, this method has the drawback that its raw materials are expensive. There is also the danger of fire or explosion due to the small particle diameter-metal powder having a large surface area, which may cause problems with cost overruns for the production equipment. Moreover, since the resultant porous body's pore diameter is reflective of the impregnated foam resin's aperture diameter, the pore diameter is several tens of μm or more.
A method is also known (JP-A-05-195110) for obtaining a metallic porous body by mixing a metal oxide powder, rather than a metal powder, with a resin binder, forming this mixture using a mold into a predetermined shape, then heating the product in an oxidizing atmosphere to burn off the resin binder and obtain a porous metal oxide-sintered body, which is then baked in a reducing atmosphere to obtain the metallic porous body. However, while this method has the advantage of allowing production of a metallic porous body having a small pore diameter of approximately 1 μm from low cost raw materials of metal oxide, the method has the drawback that the production process is cumbersome because it requires a pressure application step. In addition, the resins which can be used as the resin binder are hydrophilic resins such as polyvinyl alcohol resin, butyral resin and acryl resin. A high temperature of 1000° or more is required to completely burn off these binders which are in a pressurized state. Accordingly, this method suffers from a problem in terms of its production equipment, wherein a baking apparatus which can accommodate high temperatures is required.
Mixing a metal oxide powder with an organic binder then baking in a reducing environment without pressure molding, does allow the binder to be removed at a lower temperature, for example 700° C. when polyvinyl alcohol is used as the binder, since the organic binder is not in a compressed environment (see JP-A-2000-500826). However, because the metal oxide particles are not compressed, fusion of the metal particles obtained by reduction does not proceed, whereby only a granulized metal powder is obtained, and a metallic porous body cannot be achieved.
Thus, at present no methods are known which use a metal oxide as the raw material for obtaining a porous metal thin film at low temperature and having a small pore diameter of 1 μm or less without going through a cumbersome step such as pressurizing.
Therefore, a problem to be solved by the present invention is to provide a metal oxide dispersion that can form a thin film having high adherence to a substrate, and a process for producing a metal thin film onto a substrate which uses this metal oxide dispersion, at low cost and using a low temperature heat treatment. Another problem is also to provide a production process for obtaining a porous metal thin film.